Reduced sulfur compound oxidation by Thiobacillus caldus.

The oxidation of reduced inorganic sulfur compounds was studied by using resting cells of the moderate thermophile Thiobacillus caldus strain KU. The oxygen consumption rate and total oxygen consumed were determined for the reduced sulfur compounds thiosulfate, tetrathionate, sulfur, sulfide, and sulfite in the absence and in the presence of inhibitors and uncouplers. The uncouplers 2,4-dinitrophenol and carbonyl cyanide m-chlorophenyl-hydrazone had no affect on the oxidation of thiosulfate, suggesting that thiosulfate is metabolized periplasmically. In contrast, the uncouplers completely inhibited the oxidation of tetrathionate, sulfide, sulfur, and sulfite, indicating that these compounds are metabolized in the cytoplasm of T. caldus KU. N-Ethylmaleimide inhibited the oxidation of tetrathionate and thiosulfate at the stage of elemental sulfur, while 2-heptyl-4-hydroxyquinoline-N-oxide stopped the oxidation of thiosulfate, tetrathionate, and elemental sulfur at the stage of sulfite. The following intermediates in the oxidation of the sulfur compounds were found by using uncouplers and inhibitors: thiosulfate was oxidized to tetrathionate, elemental sulfur was formed during the oxidation of tetrathionate and sulfide, and sulfite was found as an intermediate of tetrathionate and sulfur metabolism. On the basis of these data we propose a model for the metabolism of the reduced inorganic sulfur compounds by T. caldus KU.     

Reduction of inorganic sulphur compounds by facultatively aerobic bacteria

Summary Amongst the family of the Enterobacteriaceae the ability to reduce tetrathionate to thiosulfate and thiosulfate to sulfite and sulfide occurs in the genera Proteus, Citrobacter and Salmonella. These reductions are coupled to a respiratory chain which functions under anaerobic conditions. Only during transport of electrons to tetrathionate oxidative phosphorylation has been demonstrated. Isolation and purification of the cytoplasmic membrane bound tetrathionate and thiosulfate reductase fromProteus mirabilis makes clear that this bacterium forms only one enzyme for both reductions. This enzyme has a molecular weight of 133,000 daltons and can be divided into two subunits with molecular weights of 43,000 and 90,000 daltons by treatment with sodium dodecyl sulfate and 2-mercaptoethanol. The reduction of tetrathionate is activated by its primary product thiosulfate. Nitrate or oxygen represses and inactivates the tetrathionate and thiosulfate reductase. Nevertheless the smaller subunit of this enzyme appears to be formed and assembled into the cytoplasmic membranes after anaerobic growth in the presence of nitrate. Paper read at the Symposium on the Sulphur Cycle, Wageningen, May 1974.     

Homologs from sulfur oxidation (Sox) and methanol dehydrogenation (Xox) enzyme systems collaborate to give rise to a novel pathway of chemolithotrophic tetrathionate oxidation

The SoxXAYZB(CD)₂‐mediated pathway of bacterial sulfur‐chemolithotrophy explains the oxidation of thiosulfate, sulfide, sulfur and sulfite but not tetrathionate. Advenella kashmirensis, which oxidizes tetrathionate to sulfate, besides forming it as an intermediate during thiosulfate oxidation, possesses a soxCDYZAXOB operon. Knock‐out mutations proved that only SoxBCD is involved in A. kashmirensis tetrathionate oxidation, whereas thiosulfate‐to‐tetrathionate conversion is Sox independent. Expression of two glutathione metabolism‐related proteins increased under chemolithotrophic conditions, as compared to the chemoorganotrophic one. Substrate‐dependent oxygen consumption pattern of whole cells, and sulfur‐oxidizing enzyme activities of cell‐free extracts, measured in the presence/absence of thiol inhibitors/glutathione, corroborated glutathione involvement in tetrathionate oxidation. Furthermore, proteome analyses detected a sulfite:acceptor oxidoreductase (SorAB) exclusively under chemolithotrophic conditions, while expression of a methanol dehydrogenase (XoxF) homolog, subsequently named thiol dehydrotransferase (ThdT), was found to increase 3‐ and 10‐fold during thiosulfate‐to‐tetrathionate conversion and tetrathionate oxidation respectively. A thdT knock‐out mutant did not oxidize tetrathionate but converted half of the supplied 40 mM S‐thiosulfate to tetrathionate. Knock‐out of another thiosulfate dehydrogenase (tsdA) gene proved that both ThdT and TsdA individually converted ～ 20 mM S‐thiosulfate to tetrathionate. The overexpressed and isolated ThdT protein exhibited PQQ‐dependent thiosulfate dehydrogenation, whereas its PQQ‐independent thiol transfer activity involving tetrathionate and glutathione potentially produced a glutathione:sulfodisulfane adduct and sulfite. SoxBCD and SorAB were hypothesized to oxidize the aforesaid adduct and sulfite respectively.     

Kinetics and energetics of reduced sulfur oxidation by chemostat cultures of Thiobacillus ferrooxidans

Thiobacillus ferrooxidans was grown in chemostat cultures with thiosulfate and tetrathionate as the limiting substrates. The yields at steady state on both substrates at different dilution rates were calculated. In a few experiments the air supply was supplemented with 2% CO₂ (v/v). This resulted in a slightly increased yield.Cells from the chemostat cultures were used to study the kinetics of thiosulfate, tetrathionate, sulfite and sulfide oxidation. With all substrates mentioned the Ks values were in the micromolar range. The values for thiosulfate and tetrathionate were 2 orders of magnitude lower that those published previously.     

Oxidation of Inorganic Sulfur Compounds by Obligately Organotrophic Bacteria

New data obtained by the author and other researchers on two different groups of obligately heterotrophic bacteria capable of inorganic sulfur oxidation are reviewed. Among culturable marine and (halo)alkaliphilic heterotrophs oxidizing sulfur compounds (thiosulfate and, much less actively, elemental sulfur and sulfide) incompletely to tetrathionate, representatives of the gammaproteobacteria, especially from the Halomonas group, dominate. Some denitrifying species from this group are able to carry out anaerobic oxidation of thiosulfate and sulfide using nitrogen oxides as electron acceptors. Despite the low energy output of the reaction of thiosulfate oxidation to tetrathionate, it can be utilized for ATP synthesis by some tetrathionate-producing heterotrophs; however, this potential is not always realized during their growth. Another group of marine and (halo)alkaliphilic heterotrophic bacteria capable of complete oxidation of sulfur compounds to sulfate mostly includes representatives of the alphaproteobacteria which are most closely related to nonsulfur purple bacteria. They can oxidize sulfide (polysulfide), thiosulfate, and elemental sulfur via sulfite to sulfate but neither produce nor oxidize tetrathionate. All of the investigated sulfate-forming heterotrophic bacteria belong to lithoheterotrophs, being able to gain additional energy from the oxidation of sulfur compounds during heterotrophic growth on organic substrates. Some doubtful cases of heterotrophic sulfur oxidation described in the literature are also discussed.     

Metabolism of thiosulfate and tetrathionate by heterotrophic bacteria from soil

Two heterotrophic bacteria that oxidized thiosulfate to tetrathionate were isolated from soil. The enzyme system in one of the isolates (C-3) was constitutive, but in the other isolate (A-50) it was induced by thiosulfate or tetrathionate. The apparent K(m) for oxygen for thiosulfate oxidation by A-50 was about 223 mum, but, for lactate oxidation by A-50 or thiosulfate oxidation by C-3, the apparent K(m) for oxygen was below 2 mm. The oxidation of thiosulfate by A-50 was first order with respect to oxygen from 230 mum. The rate of oxidation was greatest at pH 6.3 to 6.8 and at about 10 mm thiosulfate, and it was strongly inhibited by several metal-binding reagents. Extracts of induced A-50 reduced ferricyanide, endogenous cytochrome c, and mammalian cytochrome c in the presence of thiosulfate. A-50, once induced to oxidize thiosulfate, also reduced tetrathionate to thiosulfate in the presence of an electron donor such as lactate. The optimal pH for this reaction was at 8.5 to 9.5, and the reaction was first order with respect to tetrathionate. There was no correlation between the formation of the thiosulfate-oxidizing enzyme of A-50 and the incorporation of thiosulfate-sulfur into cell sulfur. Thiosulfate did not affect the growth rate or yield of A-50.     

Oxidation of inorganic sulfur compounds by obligatory organotrophic bacteria

New data obtained by the author and other researchers on two different groups of obligately heterotrophic bacteria capable of inorganic sulfur oxidation are reviewed. Among culturable marine and (halo)alkaliphilic heterotrophs oxidizing sulfur compounds (thiosulfate and, much less actively, elemental sulfur and sulfide) incompletely to tetrathionate, representatives of the gammaproteobacteria, especially from the Halomonas group, dominate. Some of denitrifying species from this group are able to carry out anaerobic oxidation of thiosulfate and sulfide using nitrogen oxides as electron acceptors. Despite the low energy output of the reaction of thiosulfate oxidation to tetrathionate, it can be utilized for ATP synthesis by some tetrathionate-producing heterotrophs; however, this potential is not always realized during their growth. Another group of marine and (halo)alkaliphilic heterotrophic bacteria capable of complete oxidation of sulfur compounds to sulfate mostly includes representatives of the alphaproteobacteria most closely related to nonsulfur purple bacteria. They can oxidize sulfide (polysulfide), thiosulfate, and elemental sulfur via sulfite to sulfate but neither produce nor oxidize tetrathionate. All of the investigated sulfate-forming heterotrophic bacteria belong to lithoheterotrophs, being able to gain additional energy from the oxidation of sulfur compounds during heterotrophic growth on organic substrates. Some doubtful cases of heterotrophic sulfur oxidation described in the literature are also discussed.     

Effect of enzymic assay conditions on sulfite reduction catalysed by desulfoviridin from Desulfovibrio gigas.

The type and the amount of end products resulting from sulfite reduction catalysed by a single partially purified desulfoviridin preparation from Desulfovibrio gigas were shown to depend upon the enzymic assay conditions employed. Both manometric and spectrophotometric assays were used, with reduced methyl viologen serving as the electron donor in each system. Trithionate, thiosulfate, tetrathionate and sulfide were identified as possible end products. In the manometric assays, sulfide production was favoured by high reduced methyl viologen concentrations, low sulfite concentrations and a pH value of 7.0 as opposed to 6.0. In the spectrophotometric assays, results approaching the stoichiometric conversion of sulfite to sulfide were obtained only at high initial reduced methyl viologen concentrations.     

Chemolithoautotrophic oxidation of thiosulfate, tetrathionate and thiocyanate by a novel rhizobacterium belonging to the genus Paracoccus

Two tropical leguminous-rhizospheric strains, SST and JT 001, phylogenetically closest to Paracoccus thiocyanatus and Paracoccus pantotrophus, respectively, were isolated on reduced sulfur compounds as sole energy and electron sources. While SST had versatile chemolithotrophic abilities to oxidize thiosulfate, tetrathionate, thiocyanate, sulfide and elemental sulfur, JT 001 could oxidize thiosulfate, soluble sulfide, elemental sulfur and a relatively lesser amount of tetrathionate. Positive hybridization signals were detected for JT 001 but not SST, when their genomic DNAs were probed with DIG-labeled sulfur oxidation genes amplified from the chemolithotrophic alphaproteobacterium Pseudaminobacter salicylatoxidans KCT001. Though the new isolate SST exhibited high 16S rRNA gene sequence similarity with the monotypic species P. thiocyanatus, it was found to be considerably distinct from the latter in terms of phenotypic and chemotaxonomic characteristics. Polyphasic systematic analysis, however, confirmed that JT 001 was a strain of P. pantotrophus.     

Nutritional studies with Pseudomonas aeruginosa grown on inorganic sulfur sources.

Pseudomonas aeruginosa was grown on a succinate-basal salts medium supplemented with various inorganic sulfur compounds as its sole source of sulfur. The organism was able to grow on the sodium salts of sulfide, thiosulfate, tetrathionate, dithionite, metabisulfite, sulfite, or sulfate, but not on those of dithionate. Analyses of the culture media after 24 h of growth indicated accumulation of sulfate from each inorganic sulfur source except sulfate. Manometric studies with resting cells obtained by growth on each of these sulfur sources yielded net oxygen uptake for all substrates except sulfite and dithionate. Similar results were obtained with extracts from these cells by spectrophotometric techniques. Thiosulfate oxidase activity appeared to be induced by growth on sulfide, thiosulfate, or tetrathionate, with little or no activity observed when cells were grown on inorganic sulfur sources of higher oxidative states. Metabisulfite oxidase appeared to be associated with growth on all inorganic sulfur compounds. Rhodanese activity appeared to be constitutively present, and its activity, observed only in soluble fraction, seemed independent of the growth medium employed. Thiosulfate and tetrathionate oxidase activities were studied in greater detail than some of the other sulfur oxidases, and both were found to be distributed between particulate and soluble fractions.     

The production and utilization of intermediary elemental sulfur during the oxidation of reduced sulfur compounds by Thiobacillus ferrooxidans

The intermediary production of elemental sulfur during the microbial oxidation of reduced sulfur compounds has frequently been reported. Thiobacillus ferrooxidans, an acidophilic chemolithoautotroph, was found to produce an insoluble sulfur compound, primarily elemental sulfur, during the oxidation of thiosulfate, trithionate, tetrathionate and sulfide. This was confirmed by light and electron microscopy. Sulfur was produced from sulfide by an oxidative step, while the production from tetrathionate was initiated by a hydrolytic step, probably followed by a series of chemical reactions. The oxidation of intermediary sulfur was severely inhibited by sulfhydryl-binding reagents such as N-ethylmaleimide, by the addition of uncouplers or after freezing and thawing of the cells, which probably damaged the cell membrane. The mechanisms behind these inhibitions have not yet been clarified. Finally, it was observed that elemental sulfur oxidation by whole cells depended on the medium composition. The absence of sulfate or selenate reduced the sulfur oxidation rate.Non-standard abbreviations NEM N-ethylmaleimide - CCCP carbonyl cyanide m-chlorophenyl hydrazone     

Sulfur cycling in Catenococcus thiocyclus

Abstract: Since its isolation from marine volcanic areas, Catenococcus thiocyclus has been known to be able to oxidize thiosulfate to tetrathionate, but the benefits gained from the reaction were unknown. The energy to be gained from such a reaction is so small (1 electron per mol of thiosulfate, compared with 8 electrons if the thiosulfate is oxidized to sulfate) that it seemed unlikely to be a useful metabolic reaction. However, continuous culture experiments have now revealed that C. thiocyclus is able to gain metabolically useful energy from this oxidation (biomass yields increased by approximately 20% after the addition of 7.75 mM thiosulfate to medium containing 20 mM acetate) by combining it with the chemical reduction of the tetrathionate by sulfide. The enzymes for thiosulfate oxidation appear to be constitutive. Moreover, with a suitable primary energy source (e.g. glucose), C. thiocyclus can reduce sulfur (S°) to sulfide and Fe³⁺ to Fe²⁺. A chemical reaction then generates FeS. Such reactions may have important implications for the sulfur cycle at oxic:anoxic interfaces in marine and freshwater systems.     

Regulation of reductase formation in Proteus mirabilis

Summary Prteus mirabilis can form four reductases after anaerobic growth: nitrate reductase A, chlorate reductase C, thiosulfate reductase and tetrathionate reductase. The last three enzymes are formed constitutively. Nitrate reductase is formed only after growth in the presence of nitrate, which causes repression of the formation of thiosulfate reductase, chlorate reductase C, tetrathionate reductase and hydrogenase. Formic dehydrogenase assayed with methylene blue as hydrogen acceptor is formed under all conditions.Two groups of chlorate resistant mutants were obtained. One group does not form the reductases and formic dehydrogenase. The second group does not form nitrate reductase, chlorate reductase and hydrogenase, but forms formic dehydrogenase and small amounts of formic hydrogenlyase after growth without hydrogen acceptor or after growth in the presence of thiosulfate or tetrathionate. Nitrate prevents the formation of formic dehydrogenase, thiosulfate reductase and tetrathionate reductase in this group of mutants. Only after growth with thiosulfate or tetrathionate the reductases for these compounds are formed. Anaerobic growth of the wild type in complex medium without a fermentable carbon source is strongly stimulated by the presence of nitrate. Tetrathionate and thiosulfate have no effect at all or only a small effect. The results show that in the presence of tetrathionate or thiosulfate the bacterial metabolism is fully anaerobic, as these cells also contain formic hydrogenlyase.     

Sulfide oxidation under oxygen limitation by a thiobacillus thioparus isolated from a marine microbial mat

Abstract The colorless sulfur bacterium Thiobacillus thioparus T5, isolated from a marine microbial mat, was grown in continuous culture under conditions ranging from sulfide limitation to oxygen limitation. Under sulfide-limiting conditions, sulfide was virtually completely oxidized to sulfate. Under oxygen-limiting conditions, sulfide was partially oxidized to zerovalent sulfur (75%) and thiosulfate (17%). In addition, low concentrations of tetrathionate and polysulfide were detected. The finding of in vivo thiosulfate formation supports the discredited observations of thiosulfate formation in cell free extracts in the early sixties. In a microbial mat most sulfide oxidation was shown to take place under oxygen-limiting conditions. It is suggested that zerovalent sulfur formation by thiobacilli is a major process resulting in polysulfide accumulation. Implications for the competition between colorless sulfur bacteria and purple sulfur bacteria are discussed.     

The functional role of reduced inorganic sulfur compounds in the metabolism of the microaerophilic bacterium Spirillum winogradskii

Oxidation of reduced sulfur compounds by microaerophilic sulfur bacterium Spirillum winogradskii was found to occur only concomitantly with consumption of an organic substrate and was not linked to their utilization as electron donors in energy metabolism. No enzymes of dissimilatory sulfur metabolism were found in the cells of the sulfur bacterium oxidizing thiosulfate to tetrathionate; oxidation of thiosulfate and sulfide was caused by their reaction with reactive oxygen species (ROS), mostly H2O2 produced in the course of aerobic growth. Decreased lytic effect of ROS in the presence of thiosulfate resulted in a twofold increase in the cell yield under aerobic conditions and more efficient substrate utilization. The latter effect was caused by decreased expense of energy for the biosynthesis of oxygen-protecting polysaccharides. The stimulatory effect of thiosulfate on the growth processes was due to the activation of a number of TCA cycle enzymes producing the intermediates for constructive metabolism, especially of the NADP-dependent malic enzyme. As a result of thiosulfate-induced synthesis of SH-containing cell components, the integral antioxidative activity increased 1.5-fold.     

Thiosulfate Oxidation and Tetrathionate Reduction by Intact Cells of Marine Pseudomonad Strain 16B

Levels of thiosulfate-oxidizing enzyme (TSO) and tetrathionate reductase (TTR) were measured in washed cell suspensions of a heterotrophic marine thiosulfate-oxidizing bacterium, strain 16B. TSO activity remained virtually constant in aerobically and anaerobically grown cells and was unaffected by the presence or absence of thiosulfate and tetrathionate in the growth medium. TTR was also present in cells grown aerobically and anaerobically, but its activity was threefold greater in cells cultured in media containing tetrathionate or thiosulfate. Tetrathionate appears to be the inducer of increased TTR activity in both aerobically and anaerobically grown cells. TTR (constitutive or induced) and TSO have different pH and temperature optima. Both TTR activities were unaffected by 10 mM KCN, which reversed oxygen inhibition of tetrathionate reduction. TSO was partially inhibited by 5 μM KCN and completely inhibited by 90 μM KCN. These findings and results of experiments to determine the influence of several inorganic electron donors and acceptors on TSO and TTR activities suggest that constitutive TSO and TTR represent reverse activities of the same enzyme, whereas inducible TTR is a separate enzyme used by strain 16B only for anaerobic respiration of tetrathionate. The bacterium appears well adapted to growth in environments characterized by low oxygen tension, dilute organic carbon concentrations, and the presence of a variety of reduced, inorganic sulfur compounds.     

The role of thiosulfate in sulfur metabolism of Rhodopseudomonas globiformis

Rhodopseudomonas globiformis is able to assimilate both sulfur moieties of thiosulfate. During growth on ³⁵S-labelled thiosulfate the amino acids cysteine, homocysteine and methionine were labelled. The bulk of thiosulfate, however, was oxidized to tetrathionate and accumulated in the medium. A thiosulfate: acceptor oxidoreductase was partially purified and characterized. The enzyme oxidized thiosulfate to tetrathionate in the presence of ferricyanide. A c-type cytochrome isolated from this organism was reduced by this enzyme.     

Thiosulfate oxidation by obligately heterotrophic bacteria

Thiosulfate was oxidized stoichiometrically to tetrathionate during growth on glucose byKlebsiella aerogenes, Bacillus globigii, B. megaterium, Pseudomonas putida, two strains each ofP. fluorescens andP. aeruginosa, and anAeromonas sp. A gram-negative, rod-shaped soil isolate, Pseudomonad H_w, converted thiosulfate to tetrathionate during growth on acetate. None of the organisms could use thiosulfate as sole energy source. The quantitative recovery of all the thiosulfate supplied to heterotrophic cultures either as tetrathionate alone or as tetrathionate and unused thiosulfate demonstrated that no oxidation to sulfate occurred with any of the strains tested. Two strains ofEscherichia coli did not oxidize thiosulfate. Thiosulfate oxidation in batch culture occurred at different stages of the growth cycle for different organisms:P. putida oxidized thiosulfate during lag and early exponential phase,K. aerogenes oxidized thiosulfate at all stages of growth, andB. megaterium andAeromonas oxidized thiosulfate during late exponential phase. The relative rates of oxidation byP. putida andK. aerogenes were apparently determined by different concentrations of thiosulfate oxidizing enzyme. Thiosulfate oxidation byP. aeruginosa grown in chemostat culture was inducible, since organisms pregrown on thiosulfate-containing media oxidized thiosulfate, but those pregrown on glucose only could not oxidize thiosulfate. Steady state growth yield ofP. aeruginosa in glucose-limited chemostat culture increased about 23% in the presence of 5–22 mM thiosulfate, with complete or partial concomitant oxidation to tetrathionate. The reasons for this stimulation are unclear. The results suggest that heterotrophic oxidation of thiosulfate to tetrathionate is widespread across several genera and may even stimulate bacterial growth in some organisms.     

Sulfur Species as Redox Partners and Electron Shuttles for Ferrihydrite Reduction by Sulfurospirillum deleyianum

Iron(III) (oxyhydr)oxides can represent the dominant microbial electron acceptors under anoxic conditions in many aquatic environments, which makes understanding the mechanisms and processes regulating their dissolution and transformation particularly important. In a previous laboratory-based study, it has been shown that 0.05 mM thiosulfate can reduce 6 mM ferrihydrite indirectly via enzymatic reduction of thiosulfate to sulfide by the sulfur-reducing bacterium Sulfurospirillum deleyianum, followed by abiotic reduction of ferrihydrite coupled to reoxidation of sulfide. Thiosulfate, elemental sulfur, and polysulfides were proposed as reoxidized sulfur species functioning as electron shuttles. However, the exact electron transfer pathway remained unknown. Here, we present a detailed analysis of the sulfur species involved. Apart from thiosulfate, substoichiometric amounts of sulfite, tetrathionate, sulfide, or polysulfides also initiated ferrihydrite reduction. The portion of thiosulfate produced during abiotic ferrihydrite-dependent reoxidation of sulfide was about 10% of the total sulfur at maximum. The main abiotic oxidation product was elemental sulfur attached to the iron mineral surface, which indicates that direct contact between microorganisms and ferrihydrite is necessary to maintain the iron reduction process. Polysulfides were not detected in the liquid phase. Minor amounts were found associated either with microorganisms or the mineral phase. The abiotic oxidation of sulfide in the reaction with ferrihydrite was identified as rate determining. Cysteine, added as a sulfur source and a reducing agent, also led to abiotic ferrihydrite reduction and therefore should be eliminated when sulfur redox reactions are investigated. Overall, we could demonstrate the large impact of intermediate sulfur species on biogeochemical iron transformations.     

Thiosulfate metabolism in Rhodopseudomonas palustris

The cells of the purple nonsulfur bacterium Rhodopseudomonas palustris, Nakamura strain, are capable of oxidizing thiosulfate and sulfide both under the anaerobic conditions in the light and under the aerobic conditions in the dark. Regardless of the presence of thiosulfate in the medium, the cells contain thiosulfate reductase, rodanase, thiosulfate oxidase, and sulfite oxidase. However, the capability to oxidize thiosulfate and sulfide is induced in Rh. palustris after the cells have been incubated in the presence of thiosulfate for 2--4 hours. The process of induction is related to the synthesis of protein components. Decomposition of thiosulfate in Rh. palustris when its concentration in the medium is low (2--5 mM) is accompanied with the formation of an equimolar quantity of sulfate. When the concentration of thiosulfate is higher (10--20 mM), the products of its oxidation are tetrathionate and sulfate. Therefore, the metabolic pathway of thiosulfate in Rh. palustris depends on its concentration in the medium.